Introduction to Acid–Base Reactions
Quick Notes
- Acid–base reactions involve the transfer of a proton (H+) between two species.
- Brønsted–Lowry acid = proton (H⁺) donor
- Brønsted–Lowry base = proton (H⁺) acceptor
- The conjugate acid is what the base becomes after gaining a proton.
- The conjugate base is what the acid becomes after donating a proton.
- These reactions typically occur in aqueous solution, with water acting as either an acid or a base.
Full Notes
Brønsted–Lowry Definitions
An acid–base reaction is one in which a proton (H+) is transferred from one substance (the acid) to another (the base). This model is known as the Brønsted–Lowry definition.
- Acid = H+ donor
- Base = H+ acceptor
This definition applies broadly and can explain acid–base reactions even when water is not present, but for AP Chemistry the focus is on aqueous solutions.
If you’re given a reaction, look for the species donating a proton and the one gaining it. That tells you which is the acid and which is the base. Then look at what each turns into after the proton transfer to identify the conjugate pairs.
H+(aq) and H3O+(aq)
In aqueous solutions, H⁺ ions are never found “alone" – they bond with water molecules to form hydronium ions, H₃O⁺.
When an acid molecule dissociates in water, a water molecule ‘accepts’ a H⁺ ion from the acid, becoming H₃O⁺. The H₃O⁺ ion is ‘carrying’ the H⁺ ion from the acid and is what actually reacts with any added base.
H⁺(aq) and H₃O⁺(aq) are interchangeable in chemical equations, but H₃O⁺ gives a more accurate picture of what happens in solution.
Example In aqueous solutions, water can donate or accept protons, making it both an acid and a base depending on the context:
HCl (aq) + H₂O (l) → H₃O⁺ (aq) + Cl⁻ (aq)
HCl donates H⁺ → acid
H₂O accepts H⁺ → base
H₃O⁺ is the conjugate acid of H₂O
Cl⁻ is the conjugate base of HCl
Conjugate Acid–Base Pairs
An acid–base pair consists of two species that differ by a single proton (H⁺). Every acid has a conjugate base, and every base has a conjugate acid.
General reaction:
Where
- HA = acid
- B = base
- A⁻ = conjugate base
- HB⁺ = conjugate acid
Example NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
NH₃ = base (accepts H⁺)
H₂O = acid (donates H⁺)
NH₄⁺ = conjugate acid
OH⁻ = conjugate base
Strengths of Acids and Bases
- Strong acids donate H⁺ easily → have weak conjugate bases.
- Strong bases accept H⁺ easily → have weak conjugate acids.
Water plays a neutral role in many reactions and can act as both an acid and a base depending on what it's reacting with.
Summary
- Acid–base reactions involve the transfer of H⁺ ions between substances.
- The Brønsted–Lowry model defines acids as proton donors and bases as proton acceptors.
- Every acid–base reaction has conjugate acid–base pairs.
- In aqueous solution, water often participates by donating or accepting a proton.
Understanding this model helps predict reaction direction, identify acid–base behavior in unfamiliar species, and relate structure to function in reactions.